What SDL_PumpEvents() Does Internally in SDL2

The SDL_PumpEvents() function is a crucial part of SDL's mechanism for staying synchronized with the host operating system and handling input. Think of it as SDL "checking in" with the OS.

Behind the scenes, SDL_PumpEvents() performs several key tasks:

OS Message Polling: It communicates with the underlying operating system's event or messaging system (e.g., the Windows message queue, macOS event loop, Linux X11 or Wayland events). It asks the OS if any relevant events have occurred since the last check. These OS-level events include things like:

• Keyboard key presses or releases.
• Mouse button clicks or movements.
• Joystick/game controller input.
• Window events (resize, close request, focus change, expose).
• Application events (e.g., request to quit).

Internal State Update: Based on the events received from the OS, SDL_PumpEvents() updates SDL's internal snapshot of the system's state. This includes:

• Updating the keyboard state array (used by SDL_GetKeyboardState()).
• Updating the mouse state (position, button status, used by SDL_GetMouseState()).
• Updating joystick/controller states.
• Managing window states (size, position, focus).

Event Queue Population: For many types of OS events (especially discrete ones like key presses, mouse clicks, or quit requests), SDL_PumpEvents() translates the raw OS event into an SDL_Event structure and adds (pushes) it onto the back of SDL's internal event queue.

SDL_PumpEvents() vs SDL_PollEvent()

It's important to understand the difference:

• SDL_PumpEvents(): Gathers new events from the OS and adds them to SDL's queue. It does not remove events from the queue or give them to your application directly. It's primarily about keeping SDL's internal view of the world up-to-date.
• SDL_PollEvent(): Checks the front of SDL's internal event queue. If an event is present, it copies that event into the SDL_Event variable you provide, removes the event from the queue, and returns true. If the queue is empty, it returns false. Critically, SDL_PollEvent() also calls SDL_PumpEvents() internally if it needs to check for new events.

Why is Pumping Necessary?

Even if you don't care about specific event details and aren't using SDL_PollEvent(), you still need to call SDL_PumpEvents() regularly (or let SDL_PollEvent do it for you). If you don't, SDL won't process OS messages. This leads to the window becoming unresponsive, as it never acknowledges requests from the OS like "the user clicked the close button". It also means functions like SDL_GetKeyboardState() won't reflect the current state of the keyboard, but rather the state from the last time events were pumped.

In essence, SDL_PumpEvents() is the function that ensures SDL is aware of what's happening outside of your application's C++ code within the operating system environment.